System and method for measurement report time stamping to ensure reference time

ABSTRACT

A system and method is provided that resolves a possible ambiguity in WLAN measurement reports by having a measuring entity include timer values in returned measurement reports. The time a measurement was requested to be performed and the actual time it was done by a measuring entity can be compared by the receiver to ensure that no ambiguities occurred.

The present invention relates to Local Area Network communication, andmore particularly to a system and method for time stamping a measurementreport that ensures correctness of a reference time.

In general, there are two variants of WLANs: infrastructure-based and adhoc-type. In the former network, communication typically takes placeonly between the wireless nodes, called stations (STA), and an accesspoint (AP) that is also a STA., which together form a basic service set(BSS). Communication takes place directly between wireless nodes (STAs)which form an independent basic service set (IBSS) in the ad hoc-typenetwork.

In a WLAN, measurements are requested and reported between RadioMeasurement-capable STAs. Measurements on non-serving channels arededicated measurements, requiring the measuring STA to interrupt itsnormal operation, switch channels and acquire radio measurements.Measurements on a serving channel are executed by a STA as concurrentmeasurements. While a Measurement-capable STA is responsible fordecoding and interpreting each Measurement Report Frame and assessingits impact on its own performance, the execution of some Measurementrequests is optional and may be ignored by a receiving STA if itsexecution would significantly degrade the STA's performance.

A STA may measure one or more channels itself or a STA may request otherSTAs in the same BSS or IBSS to measure one or more channels on itsbehalf. When requesting other STAs to measure one or more channels, aSTA employs a Measurement Request Frame specifying therein at least oneMeasurement Request element.

The Institute of Electrical and Electronics Engineers (IEEE) 802.11-1999standard is a protocol standard for wireless LANs (WLANs) that focuseson the media access control (MAC) and physical (PHY) layers and ishereby incorporated by reference as if fully set forth herein. In theIEEE 802.11(h) and IEEE 802.11(k) draft supplements to this standard,which are hereby incorporated by reference as if fully set forth herein,measurement requests include a time reference specifying when arequested measurement is to be started. For example, as illustrated inFIG. 1A, a Measurement Offset parameter 100 and an Activation Delayparameter 101 together specify the reference time in IEEE TGh and TGk.Activation Delay is the measurement activation delay in number of targetbeacon transmission times (TBTTs) and Measurement Offset is themeasurement offset in timer units (TUs). FIG. 1B illustrates themeasurement requests allowed.

Due to channel conditions, more than one attempt may be made tocommunicate a measurement request to a destination station (STA). At thereceiving STA, if more than one copy of the same measurement request isreceived, then the STA discards the most recently received duplicatepackets. This is according to the protocol specified in the IEEE 802.11standard.

The discard of duplicate frames at the receiving STA, according to theIEEE 802.11 protocol, may result in the requesting and receiving STAeach having a different reference beacon from which each referencesmeasurement start times. For example, if a measurement request isintended to be transmitted in one beacon period and, due to channelconditions, the recipient STA receives the frame correctly while its ACKis not received back at the transmitting STA, and if subsequently themeasurement request is successfully transmitted in another beaconperiod, then, at the transmitting and receiving stations the referencebeacons differ.

Embodiments of the present invention provide a method and system thatresolves the ambiguity in a WLAN measurement report by including timervalues in measurement reports. The time a measurement is requested to beperformed and the actual time the measurement is made can then becompared by the requestor to ensure that no ambiguity occurs. In anembodiment of the present invention for an IEEE 802.11 WLAN, timesynchronization function (TSF) timer values are included in measurementreports.

In a preferred embodiment, a Measurement Report Frame and element aremodified to contain a field with an absolute time reference. For an IEEE802.11 WLAN the absolute time reference is the value of TSF time (orpart thereof) when requested measurements start. In all embodiments,comparison is made between the intended (requested) time to start themeasurements and the time measurements actually started to ensure thatno protocol ambiguities occurred.

FIG. 1A illustrates parameters of a service primitive that requests thetransmission of a measurement request to a peer entity in a WLAN;

FIG. 1B illustrates the measurement requests allowed in a WLAN;

FIG. 2A is a simplified block diagram illustrating the architecture of abasic service set (BSS) wireless communication system wheretoembodiments of the present invention are to be applied;

FIG. 2B is a simplified block diagram illustrating the architecture ofan ad hoc-type independent basic service set (IBSS) wirelesscommunication system whereto embodiments of the present invention are tobe applied;

FIG. 3 illustrates a simplified block diagram of an access point (AP)and each station (STA) within a particular basic service set (BSS) orindependent basic service set (IBSS) according to an embodiment of thepresent invention;

FIG. 4 illustrates a Measurement Report Frame format according to anembodiment of the present invention;

FIG. 5A illustrates a Measurement Report Element format according to anembodiment of the present invention; and

FIG. 5B illustrates a Measurement Report mode field of the MeasurementReport Element illustrated in FIG. 5A, according to an embodiment of thepresent invention.

In the following description, by way of explanation rather thanlimitation, specific details are set forth such as the particulararchitecture, interfaces, techniques, etc., in order to provide athorough understanding of the present invention. However, it will beapparent to those skilled in the art that the present invention may bepracticed in other wireless network embodiments that depart from thesespecific details.

FIG. 2A is a representative infrastructure basic service set (BSS)wireless network whereto embodiments of the present invention are to beapplied. As shown in FIG. 2A, an access point (AP) 200 is coupled to aplurality of mobile stations (STA_(i)) 201, which, through wirelesslinks 202 are communicating with each other and the AP via a pluralityof wireless channels. A key principle of the present invention is toprovide a mechanism to time stamp reports of measurements taken by theSTA_(i) 201 so that protocol ambiguities can be avoided should theyoccur and, therefore, makes it possible to employ corrective actionsafter the fact. It should be noted that the network shown in FIG. 2 issmall for purposes of illustration. In practice most networks wouldinclude a much larger number of mobile STA_(i) 201. FIG. 2B is arepresentative ad hoc-type independent basic service set (IBSS) wirelessnetwork whereto embodiments of the present invention are to be applied.As shown in FIG. 2B, a plurality of mobile stations (STA i) 201communicate with each other through wireless links 202 without any AP.

Referring to FIG. 3, the AP 200 and each STA_(i) 201 within the WLAN ofFIGS. 2A and B may include a system with an architecture that isillustrated in the block diagram of FIG. 3. Both the AP 200 and eachSTA_(i) 201 may include a receiver 301, a demodulator 302, a measurementcircuit 303, a memory 304, a control processor 305, a timer 306, amodulator 307, and a transmitter 308. The exemplary system 300 of FIG. 3is for descriptive purposes only. Although the description may refer toterms commonly used in describing particular mobile stations, thedescription and concepts equally apply to other processing systems,including systems having architectures dissimilar to that shown in FIG.3.

In a preferred embodiment, the receiver 301 and the transmitter 308 arecoupled to an antenna (not shown) to convert received measurementreports and transmit desired measurement requests into correspondingdigital data via the demodulator 302 and the modulator 307,respectively. The measurement acquisition circuit operates under thecontrol of the processor 305 to the process received Measurement ReportFrames comprising measurements and corresponding time stamps or to makerequested or periodic measurements (autonomously) in which measurementsare conveyed along with corresponding time stamps in Measurement ReportFrames. FIG. 1B shows possible requestor and measurer combinations forboth IBSS and BSS IEEE 802.11 WLANs embodiments of the presentinvention. The timer 306 is used to set the time stamp in theMeasurement Report Frames that indicates the start time of themeasurement being reported.

FIG. 4 illustrates a format for a Measurement Report Frame body 400according to a preferred embodiment of the present invention. In an IEEE802.11 WLAN embodiment of the present invention, the Measurement ReportFrame body 400 uses an Action frame body format and is transmitted by aSTA 201 in response to a Measurement Request Frame or by a STA 201autonomously providing measurement information. In all embodiments, theCategory field 401 is set equal to a value indicating a RadioMeasurement category or Spectrum Management category. The Action field402 is set equal to a value indicating a Measurement Report is containedtherein. The Dialog Token field 403 is set equal to a value in acorresponding Measurement Request Frame. If the Measurement Report Frame400 is not being transmitted in response to a Measurement Request Framethen the Dialog Token field 402 is set equal to zero. In a preferredembodiment, at least one of (1) the Time Stamp field 404 of aMeasurement Report Frame contains the value of an absolute timereference at the time when the STA started measuring the first reportedmeasurement and (2) at least one Measurement Report Element 405 500 istime-stamped 504. The Measurement Report Elements field 405 contains atleast one Measurement Report Elements 500, as illustrated in FIG. 5A.The number of Measurement Report elements 500 and length of theMeasurement Report Elements field 405 in a Measurement Report Frame 400is limited by the maximum allowed medium access control (MAC) managementprotocol data unit (MMPDU) size.

Each Measurement Report Element 500 contains a measurement report of ameasurement made by a STA 201. The format of a Measurement ReportElement 500 of a preferred embodiment is shown in FIG. 5B. The ElementID field 501 is set to an identifier assigned by a correspondingmeasurement request (e.g., according to IEEE 802.11 standard it is 39for Measurement report.) The Length field 502 is variable and depends onthe length of the Measurement Report field 507. The minimum value of theLength field is 3 (e.g., reporting a Measurement Mode equal to Incapableor Refused and using a zero-length Measurement Report field). TheMeasurement Token field 503 is set equal to the Measurement Token in thecorresponding measurement request. If the Measurement Report element isbeing sent autonomously by a STA then the Measurement Token field 503 isset equal to zero. The Time Stamp field 504 is set to the value of anabsolute timer when the requested measurement is started for eachMeasurement Element reported in a Measurement Report Frame 400, which isa TSF timer value for an IEEE 802.11 embodiment of the presentinvention. It should be noted that either or both Time-Stamp fields 404and 504 can be present in a Measurement Report.

The inclusion of the actual time 504 of a Measurement Element 500increases the confidence level of the measurement and when several STAs201 report the same measurement at close but different time actual times504, this confidence level is further enhanced. In addition, thefrequency of transmitted and requested measurement reports helps guideSTAs 201 in requesting and making measurement reports, e.g., if a reportis received by a STA 201 at 1:00 and 1:05 the STA 201 can request orreport measurements at 1:10 to obtain or provide, respectively, areliable measurement of a given element(s).

While the preferred embodiments of the present invention have beenillustrated and described, it will be understood by those skilled in theart that various changes and modifications may be made, and equivalentsmay be substituted for elements thereof without departing from the truescope of the present invention. In addition, many modifications may bemade to adapt to a particular situation and the teaching of the presentinvention without departing from its central scope. Therefore it isintended that the present invention not be limited to the particularembodiment disclosed as the best mode contemplated for carrying out thepresent invention, but that the present invention include allembodiments falling within the scope of the appended claims.

1. A method for creating a measurement report (400) among a plurality ofstations (201) in a wireless local area network (WLAN), comprising thesteps of: receiving (301) by a measurement capable station (201) of saidplurality of stations (201) a measurement request for at least onemeasurement report element (500) to be made at a given time; recordingby the measurement capable station (201) said at least one measurementreport element (500); creating by the measurement capable station (201)a measurement report (400) comprising said at least one measurementreport element (500) as one of measurement report elements containedtherein (405); respectively time-stamping (404, 504) with an absolutetime reference at least one of said at least one recorded measurementreport element (500) and said measurement report (400); and transmitting(308) by the measurement capable station said created measurementreport.
 2. The method of claim 1, wherein said time-stamping stepfurther comprises the step of setting by the measurement capable stationeach said absolute time reference to a time synchronization function(TSF) timer value (306).
 3. The method of claim 1, further comprisingthe steps of: if at least one measurement element has been time-stampedwith an absolute time reference (504), determining by the measurementcapable station (201) an earliest said absolute time reference (504) ofa measurement report element, and setting by the measurement capablestation (201) said measurement report time-stamp (404) to saiddetermined earliest absolute time reference of a measurement reportelement.
 4. The method of claim 3, wherein said time-stamping stepfurther comprises the step of setting by the measurement capable station(201) each said absolute time to a time synchronization function (TSF)timer value (306).
 5. The method of claim 1, wherein said creating stepfurther comprises the step of including in said measurement report aplurality of fields formatted as one of Dialog Measurement CategoryAction Token reserved Report Elements 401 402 403 404 405 Octets: 1 1 18 variable and Measurement Dialog Time Report Category Action TokenStamp Elements 401 402 403 404 405 Octets: 1 1 1 8 variable

the fields being— a Category field (401) that is set equal to a valueindicating a Radio Measurement category or Spectrum Management category,an Action field (402) that is set equal to a value indicating ameasurement report (500) is contained therein, a Dialog Token field(403) that is set equal to a value in a corresponding measurementrequest or, if the measurement report is not being transmitted inresponse to a measurement request, is set equal to zero, an optionalTime Stamp field that contains the value of an absolute timer reference(306) at the time when the measurement capable station performed thefirst measurement of the measurement report elements (405) reported inthe measurement report (400), and a Measurement Report Elements field(405) that contains a number of said measurement report elements 500,said number being at least one, said measurement report elements field(404) having a combined length and a plurality of sub-fields and beingformatted as Element Measurement Time Measurement MeasurementMeasurement ID Length Token Stamp Mode Type Report 501 502 503 504 505506 507 Octets: 1 1 1 8 1 1 variable

the subfields being— an Element ID subfield (501) that is set to anidentifier assigned to the requested Measurement Report, a Lengthsubfield (502) that is variable with a minimum value of 3, a MeasurementToken subfield (503) that is set equal to a Measurement Token in acorresponding Measurement Request Element of a measurement request or ifthe Measurement Report Element (500) is being sent autonomously by themeasurement capable station then the Measurement Token (503) field isset equal to zero, an optional Time-Stamp field (505) that is set to thevalue of an absolute timer (306) when the reported measurement isstarted, a Measurement Mode subfield (505) is a bit field that qualifiesthe type of measurement request being made, a Measurement Type subfield(506) that is set to an identifier that identifies the type of ameasurement request, and a Measurement Report field (507) comprising atleast one Measurement Report Element (500), wherein, at least one of theMeasurement Report Time Stamp (404) and a Measurement Element ReportTime-Stamp (504) is present.
 6. The method of claim 5, wherein: the WLANis an IEEE 802.11 WLAN; the Element Id (501) is set to a uniqueidentifier specified by an IEEE 802.11 standard; each Time Stamp field(404, 504) is a time synchronization function (TSF) timer value (306);and the combined length of the measurement report elements (405) is lessthan or equal to the maximum allowed medium access control (MAC)management protocol data unit (MMPDU) size.
 7. A method for creating anautonomous measurement report (400) having at least one measurementreport element (500), among a plurality of stations (201) in a wirelesslocal area network, comprising the steps of: recording by a measurementcapable station (201) of said plurality of stations (201) at least onepre-determined measurement report element (500); optionallytime-stamping with an absolute time reference by the measurement capablestation (201) said at least one recorded measurement report element;creating by the measurement capable station an autonomous measurementreport (400) comprising said at least one measurement report element(500); optionally time-stamping with an absolute time reference of theearliest time of a measurement report element contained therein saidautonomous measurement report (400), and transmitting (308) by themeasurement capable station said autonomous measurement report (400),wherein, at least one of said autonomous measurement report time-stamp(504) and said at least one measurement report element time-stamp (404)is included in said autonomous measurement report (400).
 8. The methodof claim 7, wherein each said time-stamping step further comprises thestep of setting by the measurement capable station (201) each saidabsolute time-stamp (404, 504) to a time synchronization function (TSF)timer value.
 9. A method for ensuring correctness of a time reference ofa requested measurement among a plurality of stations (STAs) (201) in awireless local area network (WLAN), comprising the steps of:transmitting (308) by a first station (201) a request for at least onetime-stamped measurement report element to be performed at a given time;receiving (301) by a second station both the measurement request and acorresponding measurement report (400) comprising the requested at leastone measurement report element (500) and at least one time-stamp (404,504) comprising an absolute time reference of when the measurement (507)recorded therein was done; comparing by the second station (201) thegiven time of the measurement request with the at least one time-stamp(404, 504) to determine correctness of the time-stamp.
 10. The method ofclaim 9, wherein said receiving step further comprises the step ofreceiving in said measurement report (400) a plurality of fieldsformatted as one of Dialog Measurement Category Action Token reservedReport Elements 401 402 403 404 405 Octets: 1 1 1 8 variable andMeasurement Dialog Time Report Category Action Token Stamp Elements 401402 403 404 405 Octets: 1 1 1 8 variable

the fields being— a Category field (401) that is set equal to a valueindicating a Radio Measurement category or Spectrum Management category;an Action field (402) that is set equal to a value indicating ameasurement report is contained therein; a Dialog Token field (403) thatis set equal to a value in a corresponding measurement request or, ifthe measurement report is not being transmitted in response to ameasurement request, is set equal to zero; an optional Time Stamp field(404) that contains the value of a timer (306) at the time when themeasurement capable station started measuring the first reportedmeasurement; a Measurement Report Elements (405) field that contains anumber of said measurement report elements (500), said number being atleast one, said Measurement Report Elements field (405) having acombined length and a plurality of subfields and being formatted asElement Measurement Time Measurement Measurement Measurement ID LengthToken Stamp Mode Type Report 501 502 503 504 505 506 507 Octets: 1 1 1 81 1 variable

the subfields being— an Element ID subfield (501) that is set to anidentifier assigned to the requested Measurement Report, a Lengthsubfield (502) that is variable with a minimum value of 3, a MeasurementToken subfield (503) that is set equal to the Measurement Token in thecorresponding Measurement Request element or if the Measurement Reportelement (500) is being sent autonomously by the measurement capablestation (201) then the Measurement Token field (503) is set equal tozero, an optional Time-Stamp field (505) that is set to the value of anabsolute timer (306) when the reported measurement is started for eachmeasurement element reported in a Measurement Report Frame, aMeasurement Mode field (505) is a bit field that qualifies the type ofmeasurement report being made, a Measurement Type field (506) that isset to a number that identifies the type of a measurement beingreported; and a Measurement Report field (507) that contains thereported measurement, wherein at least one of the Measurement ReportTime Stamp and Measurement Element Report Time Stamps is present. 11.The method of claim 10, wherein: the WLAN is an IEEE 802.11 WLAN; theElement Id (501) is set to a unique identifier set by an IEEE 802.11standard; each said Time Stamp (504) is a time synchronization function(TSF) timer value (306); and the combined length of the measurementreport elements is less than or equal to the maximum allowed mediumaccess control (MAC) management protocol data unit (MMPDU) size.
 12. Themethod of claim 10, wherein said receiving (301) step further comprisesthe step of receiving a measurement report having each said Time Stamp(404, 405) set using a time synchronization function (TSF) timer value(306).
 13. An apparatus configured for resource measurement among aplurality of stations in a wireless local area network (WLAN),comprising: a receiver (301) for receiving an incoming signal; ameasurement acquisition circuit (303) that measures resources of saidincoming signal received therein as at least one measurement reportelement (500); a timer (306) that provides an absolute time reference; acontrol processor (305), coupled to said measurement acquisition circuit(303) and said timer (306) and beginning at a predetermined absolutetime, configured to acquire at least one measurement report element ofsaid incoming signal and optionally associate one of (1) an absolutetime reference (306) of the start of the first measurement reported(500) within a measurement report (400) as a time-stamp (404) and (2) anabsolute time reference (306) of the start of each measurement reportelement reported therein (507) with a measurement report elementtime-stamp (504).
 14. The apparatus of claim 13, further comprising: amemory (304), coupled to said control processor (305) to store saidobtained measurement report elements (500) and optionally saidassociated measurement report element time-stamps (504); and wherein,said control processor (305) is further configured to compare thepredetermined absolute time with at least one said absolute time-stamp(404, 504) to determine correctness of said at least one absolutetime-stamp (404, 504).
 15. The apparatus of claim 14, further comprisinga transmitter that transmits a measurement report comprising a pluralityof fields formatted as one of Dialog Measurement Category Action Tokenreserved Report Elements 401 402 403 404 405 Octets: 1 1 1 8 Variableand Measurement Dialog Time Report Category Action Token Stamp Elements401 402 403 404 405 Octets: 1 1 1 8 variable

the fields being— a Category field (401) that is set equal to a valueindicating a Radio Measurement category or Spectrum Management category;an Action field (402) that is set equal to a value indicating ameasurement report is contained therein; a Dialog Token field (403) thatis set equal to a value in a corresponding measurement request or, ifthe measurement report is not being transmitted in response to ameasurement request, is set equal to zero; an optional Time Stamp field(404) that contains the value of a timer (306) at the time when themeasurement capable station started measuring the first reportedmeasurement; a Measurement Report Elements 405 field that contains anumber of said measurement report elements (500), said number being atleast one, said Measurement Report Elements field (405) having acombined length and a plurality of subfields and being formatted asElement Measurement Time Measurement Measurement Measurement ID LengthToken Stamp Mode Type Report 501 502 503 504 505 506 507 Octets: 1 1 1 81 1 variable

the subfields being— an Element ID subfield (501) that is set to anidentifier assigned to the requested Measurement Report, a Lengthsubfield (502) that is variable with a minimum value of 3, a MeasurementToken subfield (503) that is set equal to the Measurement Token in thecorresponding Measurement Request element or if the Measurement Reportelement (500) is being sent autonomously by the measurement capablestation (201) then the Measurement Token field (503) is set equal tozero, an optional Time-Stamp field (505) that is set to the value of anabsolute timer (306) when the reported measurement is started for eachmeasurement element reported in a Measurement Report Frame, aMeasurement Mode field (505) is a bit field that qualifies the type ofmeasurement report being made, a Measurement Type field (506) that isset to a number that identifies the type of a measurement beingreported; and a Measurement Report field (507) that contains thereported measurement, wherein at least one of the Measurement ReportTime Stamp and Measurement Element Report Time Stamps is present. 16.The apparatus of claim 15, wherein: the WLAN is an IEEE 802.11 WLAN; theElement Id (501) is set to a unique identifier specified by an IEEE802.11 standard; each said Time-Stamp (404, 504) is a timesynchronization function (TSF) timer value (306); and the combinedlength of the measurement report elements is less than or equal to themaximum allowed medium access control (MAC) management protocol dataunit (MMPDU) size.
 17. The apparatus of claim 15, further comprising areceiver (301) for receiving a measurement request comprising at leastone measurement request element to be measured and reported as ameasurement report element (500) and the predetermined time to startmeasuring.
 18. The apparatus of claim 17, wherein said measurementrequest is transmitted by a station (201) of said plurality of stations(201).
 19. The apparatus of claim 17, wherein said measurement requestis transmitted by an access point (200) of said plurality of stations(201).